Wolff's Law states that bone grows and remodels in response to the forces that are placed upon it. Throughout life bone is constantly remodeled by the coordinated action of bone-resorbing osteoclasts and bone-forming osteoblasts in basic multicellular units. This continuous remodeling likely serves to prevent and remove fatigue-related micro-damage and allows adaptation of the bone mass and structure. In a musculoskeletal system, the biomechanical environment plays a key role in repairing, maintaining, and remodeling of bone to meet its functional demands. After injury to bone, placing specific forces, in specific therapy frequencies can stimulate and accelerate the natural bone repair process to help the bone remodel and become healthy, normal bone again.
For several decades, clinicians and researchers have been investigating the relationship between the mechanical environment near bone repair sites and the speed of secondary bone healing. A growing body of evidence derived from animal models and clinical trials now suggests that dynamic forces can be key in promoting and accelerating the bone repair process. Certainly, the ability to promote and speed the rate of bone healing would provide significant benefits to patients and the healthcare industry in general. Approximately six million extremity fractures occur each year in the United States. Five to ten percent of these fractures will go on to delayed or non-union fractures, adding further burden to the healthcare system. Spinal fusion is surgery to permanently connect two or more vertebrae in your spine, eliminating motion between them. Spinal fusion involves techniques designed to mimic the normal healing process of broken bones. During spinal fusion, bone or a bone-like material is placed within the disc space between two vertebral bodies to fuse the vertebral levels together into a single bony element. Delayed or non-union fractures are even more common in spinal fusion procedures, with some reports suggesting up to a third of these cases don't adequately fuse. There are also various other conditions in which osteoporotic or poor quality bone becomes subject to fractures, deformities, and/or heals slowly. In the aforementioned conditions of fracture healing, vertebral fusion, and/or osteoporotic/poor quality bone, patients and healthcare providers alike are relying on a successful bone repair process to occur. However the natural bone repair process isn't always successful, and often proceeds at an excessively slow rate. Evidence suggests dynamic forces can be applied to injured bone regions to stimulate and accelerate the bone repair process.
There is an existing class of medical devices referred to as bone stimulators. They are tools that aid in bone healing and recovery, especially in the instance of delayed or non-union. The technology behind the bone stimulators is summarized into five main groups based on modes of action. These types are summarized in Table 1 below.
TABLE 1Types-Bone StimulatorsMode of Action1InductiveA wire coil that creates an electromagnetic field.2Direct CurrentA power source delivers a constant current to the desired site.3 CapacitiveAn electrical field generated between two electrodes placed at opposite ends of the treatment site.4 MagneticLow energy magnetic fields applied either statically or dynamically (pulsed)5UltrasonicLow intensity, pulsed ultrasonic signal to fracture site.
Table 2 shown in FIG. 1 illustrates some commercial marketed bone stimulator devices. These technologies are intended to reduce incidence of delayed or non-union fractures and/or fusion, improve the rate of bone healing, and shorten fracture healing times. These are typically home health care devices, in which the patient is trained how to operate and administer the device. None of these commercial embodiments are utilizing dynamic force delivery to stimulate bone repair. Furthermore, the existing bone stimulators are expensive devices, often costing several thousands of dollars. It would be beneficial to the healthcare industry to provide a lower cost approach to stimulating the bone repair process.
The existing commercial art is utilizing electrical, magnetic, and ultrasonic modes to stimulate bone cells. These modes of action are favorable in that they typically can be delivered non-invasively. For example, ultrasound signals, electrical signals, and magnetic fields are known to readily penetrate human tissue. In this manner, the bone stimulator devices can be placed in the general vicinity of the desired bone region, and the stimulator signal can penetrate into the body. However, as previously summarized, there is a large body of evidence suggesting dynamic forces (not electrical, magnetic, or ultrasonic signals) are key ingredients in stimulating bone repair and growth. The existing types of bone stimulators are not utilizing dynamic forces as a mode of action.
There are various challenges in delivering dynamic forces to a desired bone region. For example, typically it is best to apply forces directly to the bone. Challis et al. in U.S. Patent Publication No. 2005/0043659 A1 discloses a pressure cuff that non-invasively delivers compressive forces to a desired lone bone extremity. Research has shown that the magnitude and frequency of the force application must be finely controlled to aid the bone repair process. Too much force can be detrimental to bone repair, and too little will not stimulate the repair process. Non-invasive approaches like Challis must contend with delivery through a wide array of soft tissues, with huge variability patient to patient. This makes it nearly impossible to finely control the final force magnitude that actually reaches the desired bone area. Therefore, applying forces by directly coupling to the bone region in need of repair is a preferred embodiment for appropriate control of force delivery. However, various scenarios exist where it is not possible to directly couple to a bone region in need of repair or growth. Often the injury itself makes it prohibitive to directly stimulate to the injured bone. For example, in the instance of bone fractures, because of the trauma, inflammation, and subsequent cellular healing activities occurring at the fracture site, directly coupling to the fracture site would be difficult, painful, and disruptive to the cellular healing process. In the instance of vertebral fusion, the bone repair site is sealed within the vertebral disc space. Therefore it is not feasible, without extreme complexity and risk, to directly couple to the bone repair region within the disc space. In the instance of osteoporotic or poor quality bone, the bone integrity may no be adequate for directly coupling the force delivery apparatus.
Furthermore, in addition to the challenges of directly coupling to a bone region in need of repair, there are further challenges in transmitting the appropriate dynamic forces into a desired bone region in need of repair. Various scenarios may exist where the location, access constraints, or other anatomical obstacles within the body can prohibit force transmission to a desired remote bone region in need of repair. There is also research evidence that suggests, in addition to the magnitude and frequency of dynamic force application, the direction of force application to the injured bone is also an important variable for stimulating bone repair. For example, in the instance of bone fractures, forces that generally compress the fracture site are thought to be more beneficial than forces that shear the fracture site.
There is a large body of scientific and clinical evidence suggesting dynamic forces are key ingredients in stimulating bone repair and growth. Existing commercial art has not focused on force delivery to a desired bone area; rather existing art is utilizing electrical, magnetic, and ultrasonic modes to stimulate bone cells. Dynamic force delivery to a desired bone region could be utilized, for example, to promote fracture healing, treat osteoporotic or other poor quality bone, and promote vertebral fusion in conjunction with a spinal fusion procedure. There are various challenges in directly coupling to a bone region in need of repair. There are also various challenges in transmitting forces into the bone region in need of repair. Despite the challenges, there is a large body of evidence suggesting it would be beneficial to develop new art that focuses on delivering force stimulation to remote bone areas in need of repair. Furthermore, it would be beneficial to accomplish such an orthopedic apparatus in a simple cost effective manner, and thereby allow the technology to be applied as a cost savings adjunct, rather than an expensive secondary treatment option.